Colloidal Atomic Layer Deposition enables Synthesis of Nano-Heterostructures with Atomic Layer Precision

In contrast to molecular systems, which are defined with atomic precision, nanomaterials generally show some heterogeneity in size, shape, and composition. The sample inhomogeneity translates into a distribution of energy levels, band gaps, work functions, and other characteristics. The lack of atomistic control during nanomaterial synthesis also limits our ability to perform “total-synthesis” of sophisticated nano-heterostructures with precisely arranged multiple components and fine-tuned properties. We discuss a general synthetic strategy which largely circumvents these limitations of traditional colloidal synthesis. Colloidal Atomic Layer Deposition (c-ALD) allows significant reduction of inhomogeneity for nanomaterials without compromising their structural perfection. We report a novel realization of c-ALD step sequence which significantly improves synthetic control and quality of synthesized nanomaterials. In traditional gas-phase ALD, the substrate and gaseous reactants act as the stationary and mobile phases, respectively. Such distinction facilitates removal of unreacted precursors by pulsing inert gas after each half-reaction. For c-ALD, we inverted the stationary and mobile phases – reactants form the stationary phase while the substrate is moved in and out of the reactor as the mobile phase. This approach brings c-ALD closer to traditional ALD and is expected to make it a similarly powerful and versatile technique. Our improved c-ALD enables synthesis of epitaxial nano-heterostructures of unprecedented complexity, ultimately enabling bandgap and strain engineering in colloidal nanomaterials synthesized with close-to-atomistic accuracy. Improved synthetic control elucidates the effects of quantum confinement and strain on the properties of semiconductor nanostructures.